linux-kernel - Re: [PATCH] clocksource: Warn if too many missing ticks are detected

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Date:   Tue, 18 Sep 2018 17:54:16 -0400
From:   Waiman Long <longman@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     John Stultz <john.stultz@...aro.org>, linux-kernel@...r.kernel.org,
        Stephen Boyd <sboyd@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] clocksource: Warn if too many missing ticks are detected

On 09/18/2018 05:07 PM, Thomas Gleixner wrote:
> Waiman,
>
> On Tue, 18 Sep 2018, Waiman Long wrote:
>
>> The clocksource watchdog, when running, is scheduled on all the CPUs in
>> the system sequentially on a round-robin fashion with a period of 0.5s.
>> A bug in the 4.18 kernel is causing missing ticks when nohz_full
>> is specified. Under some circumstances, this causes the watchdog to
>> incorrectly state that the TSC is unstable because of counter overflow
>> in the hpet watchdog clock source after a few minutes delay.
>>
>> That particular bug is fixed by the 4.19 commit 7059b36636beab ("sched:
>> idle: Avoid retaining the tick when it has been stopped"). To make it
>> easier to catch this kind of bug in the future, a check is added to see
>> if there is too much delay in the watchdog invocation and print a
>> warning once if it happens.
> I like the idea.
>
>> Signed-off-by: Waiman Long <longman@...hat.com>
>> ---
>>  kernel/time/clocksource.c | 13 +++++++++++++
>>  1 file changed, 13 insertions(+)
>>
>> diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
>> index 0e6e97a..2ea5db0 100644
>> --- a/kernel/time/clocksource.c
>> +++ b/kernel/time/clocksource.c
>> @@ -140,6 +140,7 @@ static void inline clocksource_watchdog_unlock(unsigned long *flags)
>>   * Interval: 0.5sec Threshold: 0.0625s
>>   */
>>  #define WATCHDOG_INTERVAL (HZ >> 1)
>> +#define WATCHDOG_INTERNVAL_NS (NSEC_PER_SEC >> 1)
>>  #define WATCHDOG_THRESHOLD (NSEC_PER_SEC >> 4)
>>  
>>  static void clocksource_watchdog_work(struct work_struct *work)
>> @@ -242,6 +243,18 @@ static void clocksource_watchdog(struct timer_list *unused)
>>  		wd_nsec = clocksource_cyc2ns(delta, watchdog->mult,
>>  					     watchdog->shift);
>>  
>> +		/*
>> +		 * When the timer tick is incorrectly stopped on a CPU with
>> +		 * pending events, for example, it is possible that the
>> +		 * clocksource watchdog will stop running for a sufficiently
>> +		 * long enough time to cause overflow in the delta
>> +		 * computation leading to incorrect report of unstable clock
>> +		 * source. So print a warning if there is unusually large
>> +		 * delay (> 0.5s) in the invocation of the watchdog. That
>> +		 * can indicate a hidden bug in the timer tick code.
>> +		 */
>> +		WARN_ON_ONCE(!wd_nsec || wd_nsec > 2*WATCHDOG_INTERNVAL_NS);
> But this is using the watchdog delta to check. If that wrapped the
> detection is broken.
>
> I'd rather use watchdog_timer.expires and check against jiffies. That tells
> you how late the timer callback actually is and does not suffer any
> wraparound issues.

The clocksource_delta() function will deal with wrap-around in the
counter value. It is only when the counter advances more than 0x80000000
for 32-bit hpet counter mask that a value of 0 will be returned. That is
why I have a !wd_nsec check there. There is a small chance when the
warparound is just within the 1 second window that the test fails. In
this case, the following kind of warning will certainly be triggered:

[  578.890937] clocksource: timekeeping watchdog on CPU21: Marking
clocksource 'tsc' as unstable because the skew is too large:
[  578.890938] clocksource:                       'hpet' wd_now:
ee332105 wd_last: 544f80e7 mask: ffffffff
[  578.890939] clocksource:                       'tsc' cs_now:
4b6e6ccb5d609 cs_last: 4b679a469d09e mask: ffffffffffffffff
[  578.890954] tsc: Marking TSC unstable due to clocksource watchdog
[  578.890963] TSC found unstable after boot, most likely due to broken
BIOS. Use 'tsc=unstable'.
[  578.890965] sched_clock: Marking unstable (578920214163,
-28725675)<-(579047174801, -156217937)
[  578.891056] clocksource: Switched to clocksource hpet

Another reason that I used wd_nsec is because the data has already been
computed.

I am perfectly fine to use the watchdog_timer.expires as suggested, though.

Cheers,
Longman

watchdog_timer.expires